The invention relates to a method of claim 1 and a device for producing a fiber web consisting of cellulose fibers for use in hygiene products, in particular, personal, absorbing hygiene products. The invention further relates to an absorbent fiber web manufactured according to this method.
It is known to combine cellulose-containing material such as wood or plant fibers into a fiber web by employing a combination of mechanical and chemical processing steps under intensive heating while excluding oxygen. The aim of such a process is to avoid the use of binding agent additives either completely or to a large extent. According to one of these known methods, disclosed in U.S. Pat. No. 4,111,744, cellulose fibers with a moisture content of 3 to 12 percent by weight are subjected to pressure in an oxygen-free atmosphere at a temperature of 450 to 800xc2x0 F. (=232 to 426xc2x0 C.), which is a high temperature environment beyond the cellulose carbonizing temperature and cellulose combustibility temperature. Paper-type products may also be manufactured using the aforementioned known method, but, only that of stiff cardboard.
The disadvantage of this method is that a considerable technological effort needs to be invested to heat the pressurized space and to prevent combustion of the material through oxygen-free manufacturing.
Also known from the International Patent Publication No. WO 94/10956 is a method for producing under pressure absorbent web products from dry cellulose fibers and additives by compressing a material with a weight per unit area of 30-2000 g/cm2 to a product with a density of 0.2-1.0 g/cm3. Compressing is carried out using smooth calender rollers. The disadvantage of this method is that although the density is increased, the tear strength of the material itself is low. Synthetic additives, especially theromplasts, must be added to increase the tear strength.
It is further known from the U.S. Pat. No. 3,692,622 to initially form an irregular cellulose fiber layer and under relatively low pressure to produce a loose non-woven fabric with a low density and tear strength. The loose non-woven is then entered into the gap of an additional pair of calender rolls and embossed with a pattern of point- or line-shaped pressure zones. The result is a soft, absorbent web material with a base weight of about 16.9 to 50.9 g/m2. The tear strength of this fiber web is about 0.09 kN/m. Thus, it is a material that tears easily as is the case with facial tissues, for example. The calender pressures applied for this known product are about 2,000 to 10,000 psi corresponding to 14 to 69 MPa. The US-A document speaks of resultant hydrogen bonding, as is also the result in self-bonding conventional paper products.
The fiber web manufactured by the method is said to be particularly suitable for manufacturing hygiene products. It is said to be very absorbing, soft and capable for processing as a web. Single-use hygiene articles such as diaper panties and such are manufactured in high volume. The core absorbing layers used for these products should be tolerated well by the body, the absorbed liquids well distributed, and after use, the products should rot in landfills without residue. A known method is to manufacture the absorbing layer of a wood cellulose fiber matrix, where so-called superabsorbers can be added to this fiber matrix to increase the liquid absorption capacity. Superabsorbers are polymers that can absorb water by building hydrogels.
It is a principal object of the present invention to specify a method for producing a fiber web made of cellulose fibers, where essentially no binding agents need to be used, and were the process can be carried out at room temperatures under normal atmospheric pressure and with the oxygen content of ambient air.
This object, as well as other objects which will become apparent from the discussion that follows, are accomplished with the method for manufacturing a fiber web made of cellulose fibers, which is largely tear resistant, absorbent and rollable, using the following processing steps
(a) placing an irregular cellulose fiber layer and pre-condensing it under relatively low pressure to produce a loose non-woven with low density and tear strength; and
(b) inserting the loose non-woven into the gap of a pair of calender rolls that is used to create a pattern of point- or line-shaped pressure zones under relatively high pressure, where the irregularly arranged fibers are pressed onto each other, wherein
(1) the moisture content of the loose non-woven is up to 5 percent by weight when it is inserted,
(2) the irregularly arranged fibers are pressed onto each other in a second pair of calender rolls under a pressure in a range between 250 and 600 MPa such that a non-separating fusion of the fibers occurs and a fiber web with an embossing pattern is created, and
(3) the tear strength of the fiber web is at least 0.12 kN/m.
It is assumed that in the technology of producing cellulose fibers it is known to make them of a wood derivative known in the industry as xe2x80x9cfluff pulpxe2x80x9d. This material is a standardized wood product made of cellulose material shipped in boards or webs, so-called wood pulp cardboards, where said material is crushed in a hammer mill and separated into fibers until it turns into a cotton-like product of cellulose fibers, namely fluff pulp. A description of such a standardized crushing process can be found, for example, in the brochure of the company Dan-Webforming International A/S. Risskov, Denmark.
This wood derivative called xe2x80x9cfluff pulpxe2x80x9d is a product that is used in large quantities in the so-called water-less paper production. Preferably, the fibers have a length of about 1 to 5 mm as they exit the hammer mill. According to the first step of the aforementioned process, they are embedded irregularly in a cellulose fiber layer with a height of 5 to 15 mm and are preferably sent on a conveyor belt to a movable strainer through a pre-condenser station that consists preferably of a pair of calender rolls with low pressure, such that the result is a loose non-woven with low density and tear strength. The tear strength is dimensioned such that the non-woven can sag over a length of 0.1 to 1 m without tearing. It can also withstand air pressures that occur during the production.
This essentially known and still very loose non-woven is inserted into a gap of a pair of calender rolls, where a significant pressure is applied in the point-shaped pressure zones. The pressure must be at least 100 and should be about 520 MPa (MPa=N/mm2). The liquid limit of the material used for the rollers is generally the upper pressure limit. According to the state-of-the-art, such high pressures have not been used until now. To produce such a pressure, rollers may be used with studs, with line patterns offset from one another or with other protruding point- or line-shaped pressure surfaces, where the array density of the point-shaped pressure zones is between 1 and 16 array points per cm2.
A fiber web, preferably with a m2 weight between 50 g and 1500 g, is produced according to the method. Due to the distribution of the connecting points, this new fiber web has become so strong that a tear strength of at least 0.12 kN/m, preferably of up to 0.65 kN/m, is achieved. The thickness of the fiber web is dependent on the desired metrage.
The size of the pressure area of the point-shaped pressure zones is dependent on the pressure that can be achieved between the second calender rollers. Point-shaped pressure zones with areas between 0.05 and 10 mm2 have proven sufficient.
As has already been emphasized, the temperature of the second pair of calender rolls should be maintained at room temperature, that is, between 19 and 25xc2x0 C. The operation can also take place at higher temperatures. It should be noted that the temperature will increase in the pressure zones due to the significant use of power.
Pre-compression should take place at a tool temperature of between 18 and 320xc2x0 C., preferably between 250 and 300xc2x0 C. Preferably, the pre-compression tool is a pair of calender rolls that can be heated.
The fiber and/or the loose non-woven are brought to a certain moisture content before entering the calender rolls, where preferably the moisture content should be set to between 2 and 9 percent in weight, at a minimum to 1.5 percent in weight.
Starting material is the aforementioned fluff pulp wood derivative. Preferably, this is a standardized defibered product, such as the one also used in manufacturing fiber webs according to known methods. Sulfite or sulfate bleached long fiber cellulose of northern wood appears very advantageous.
It has also proven advantageous, when the cellulose fibers were not bleached to total whiteness but instead when they still contained a certain content on natural wood materials. The degree of whiteness should be between 80 and 92%, preferably between 85 and 89%. A certain remaining lignin content has shown to be advantageous as well, for example if it is between 0.5 and 5 percent in weight of the starting material.
Non-binding, inorganic pigments or fillers, such as titanium oxide, kaolin or zeolithe can be added to the starting material.
A certain amount of superabsorbers can be added to the starting fibers as well, where the acrylate composites known as superabsorbers can be added in powder form to the fluff pulp in an amount of, for example, 0.5 to 70 percent in weight (in relation to the total amount) and where the manufacturing process is not significantly influenced by this.
In the pressure zone of the second calender roll, the radial distance of the calender roll pair beyond the actual point-shaped pressure zones should be about 1 to 15 mm such that the material beyond the pressure zone is not squashed during the pressure application, but is rather fluffed and somewhat compressed.
The gap in the pressure zone of the second pair of calender rolls is dependent on the metrage and the thickness of the inserted loose non-woven. In general, the gap should not exceed a clear width of 0.05 to 1 mm.
A significant part of the device for carrying out the method is formed by the second pair of calender rolls, which is preferably made up of two steel calender rollers both provided with numerous studs distributed across the outer surfaces of the rollers corresponding to point-shaped pressure zones that are surrounded by indentations that exhibit a multiple of the volume of the raised areas. In the operating gap, the raised areas of the two rollers are opposite one another, and a pressure of at least 200 MPa up to the maximum liquid limit of the material used for the studs is exerted on the non-woven located in the point-shaped pressure zones.
The preferable height of the studs or other pressure zones is between 0.5 and 15 mm from the roller base. The studs are preferably shaped as pyramids or truncated cones with a stud coat angle of 10 to 45xc2x0 in relation to the radius. Line-shaped or similar pressure zones are possible as well.
The irregularly arranged fibers are compressed under very high localized pressure in line or point-shaped pressure zones, such that a multitude of close fusions of the fiber bodies occur that will not separate after the pressure is released. A product of numerous irregular cellulose fibers is produced, where said fibers are connected in the pressure zones through fiber bonding. The fiber web has sufficient tear strength and also a high absorption capacity such that it is ideally suited for hygiene products.
It has shown that in order to meet the specific requirements of the hygiene industry, the web of fiber materials must subsequently be combined with suitable materials in a labor-intensive manner. Thus, the additional objective is given to specify an additional method for producing a fiber web consisting of cellulose fibers that is equipped with, for example, increased tear strength, density or breathing and/or insulating capacities.